Conventional “optical” microlithography systems utilize respective deep-ultraviolet wavelengths of light as the lithographic energy beam, such as “i-line” radiation from a mercury lamp or light produced by an excimer laser such as a KrF or ArF excimer laser. The reticle (pattern-defining master plate) used in optical microlithography typically is made of glass, has a square or other rectilinear plan profile, and has a thickness of several mm. Since optical microlithography currently is the “workhorse” microlithography technology used widely throughout the world, reticle-manipulating devices in common use are configured for use with an optical-lithography reticle.
In recent years, substantial engineering effort has been directed to the development of a practical “next-generation” microlithography system that offers prospects of producing finer pattern-transfer resolution than currently obtainable using optical microlithography. One attractive next-generation lithography (NGL) approach involves the use of a charged particle beam, such as an electron beam or ion beam, as the lithographic-energy beam. A key challenge in the development of a practical electron-beam microlithography system is configuring the system to produce the desired fine-ness of pattern-transfer resolution without sacrificing “throughput” (number of units, such as semiconductor wafers, that can be exposed lithographically by the system per unit time).
In an electron-beam (EB) microlithography system (as an example of a CPB microlithography system), the thick, square, glass reticle conventionally used for optical microlithography is not used. Instead, the EB-lithography reticle typically is round (e.g., 200 to 300 mm in diameter) and much thinner (e.g., 0.5 to 1.0 mm). These reticles are made from silicon wafers (“reticle substrates”) having a standardized configuration (e.g., a particular diameter, thickness, notched, or non-notched) according to standards established by Semiconductor Equipment and Materials International (SEMI). Almost the entire surface of the EB-lithography reticle is patterned. Since the entire pattern cannot be exposed in a single exposure “shot,” the EB lithography reticle is divided into multiple “exposure units” (usually termed “subfields”) each defining a respective portion of the pattern and being individually exposed. During exposure an electron beam is irradiated, from above, onto a selected subfield of the reticle.
Portions of the reticle that define pattern features and that actually are irradiated by the electron beam are membranous and hence very thin and delicate. Consequently, these portions of the reticle must not contact any other surfaces (such as a surface of a reticle manipulator). Rather, during reticle manipulation, the reticle must be handled and supported only by its non-patterned (and more robust) peripheral “handling zone.” The handling zone of an EB-lithography reticle typically is narrow, with a maximum usable width of several mm.
Unfortunately, among conventional reticle “manipulators” (encompassing any of various devices, usually robotic, that perform handling and/or moving of the reticle), none are configured to accommodate a thin, circular, reticle or reticle substrate as used in EB microlithography or CPB microlithography in general.